Conventional seismic methods for exploring subterranean strata beneath the seabed involve generating a seismic wave and measuring the response from the subsurface. The seismic wave may be simple or complex and may be generated at sea level, beneath the surface of the water or at the seabed. The response is detected by a series of spaced receivers which are commonly positioned on cables towed behind an exploration vessel or within nodes positioned on the sea floor. Generally, the receivers are held stationary for the detection step and are then moved to a different location and the process is repeated.
The response to a seismic event in the solid rock at the sea floor includes a compression wave (P-wave) and a shear wave (S-wave). P-waves are considered well suited to imaging structures while the combination of S-waves is well suited to determining rock and fluid characteristics. P-waves travel through rock and sea water while S-waves travel through rock only. Thus, if the receivers are hydrophones floating at or beneath the surface, they will detect only the P-waves. In order to detect the S-waves, it is necessary to use geophones located at the seabed.
It has also been recognised that better seismic imaging can be achieved by making use of both P- and S-waves. However, the costs involved in positioning and re-positioning geophones on the sea bed has been found to be prohibitively costly. This is particularly so since in order to detect S-waves effectively, three independent orthogonal and stationary geophones are required at each recording location.
4C seismic imaging of the subsurface may add more and better information to exploration due to high quality recording of shear waves (S-waves) at the water bottom. Unfortunately, 4C-imaging has suffered from a combination of extreme high acquisition cost, variable payback and uncertainties in the prediction of payback.
It has been recognised that the cost effectiveness of carrying out such seismic imaging, and in particular S-wave measurements, could be greatly reduced by avoiding the need to locate detection apparatus at the seabed. Thus, it would be desirable to measure an S-wave from a position spaced from the seabed and so allow effective re-positioning of the detection apparatus with respect to the seabed. However, as mentioned, S-waves do not travel through sea water, making direct sensing remote from the seabed impossible using traditional techniques. Remote sensing has further inherent problems in that the detection apparatus is subjected to ocean currents which can inhibit effective positioning of the detection apparatus, and introduce noise into measurements, making correlation of the results very difficult.
It is therefore an object of the invention to allow seismic exploration in which both P-waves and S-waves are detected but without the disadvantages of known techniques.
A large range of interferometric techniques and instruments exist for measuring parameters such as distance, topography, dimensions, displacements and vibrations. Many of these interferometers are based on the use of a laser, where a detector or a detector array is illuminated with laser light reflected from the object under investigation and also illuminated by a so called reference beam. As the two beams interfere, different factors such as object displacements can be detected and quantified, as the nature of the interference gives information relating to such parameters.
A commonly used technique is the so called “laser doppler velocimetry” (LDV). With this technique, the displacements or vibrations of a single point on the object under investigation (OUI) are measured, as the laser light reflected from this point undergoes a quantified shift in wavelength as the object is moved with a given velocity. Some LDV systems can also be used in a scanning mode, where displacements or vibrations can be mapped over a full field.
Some other types of interferometers are being used for the testing of optical components and other smooth surfaces. In such cases, the light reflected from (or transmitted through) the object under investigation (object light) has a more or less flat or spherical phase front, and when this wave is combined with an expanded reference beam, interference fringes appear and these can be used to analyse surface properties of the object under investigation.
When interferometers such as LDV's or other interferometers are used to measure displacements of rough surfaces, the light reflected from the object surface will have a speckle nature, where the regular flat or spherical wave front is degenerated to a chaotic spatial pattern with varying intensity and phase. This speckle pattern is the result of interference between many waves (wavelets) reflected from different microscopic points on the rough object surface.
When interferometric measurements are carried out on different types of mechanical components, buildings, or even on the ground (seismic waves and earthquakes), speckle light from the object must be used for the measurement instead of a smooth wave as with testing of optical surfaces. This normally requires that the speckle pattern must not move or change in space or on the detector surface, which means that a good stability is normally required between the illumination/detector system (interferometer) and the object under investigation. Transverse movements and object tilt in particular will make the speckle pattern move and change on the surface of the detector or the detector array. Some stability problems may be overcome by using very fast detector systems, but generally, lack of stability represents a major problem to interferometric measurements.